tan 64 degrees 6'

You could just put it in as 

tan(64+6/60)=

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}{\left({\mathtt{64}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{6}}}{{\mathtt{60}}}}\right)} = {\mathtt{2.059\: \!418\: \!717\: \!851}}$$

There are 60 minutes in a degree, so 6 minutes is .1 of a degree.

So we have tan 64.1 degrees = 2.059418717851